C'KYPTODIRA. 17 



also by iho basioccipital. The three elements are usually distinct 

 in the occipital condyle. 



The outer border of the tympanic cavity is never completely 

 encircled by bone, at least a wide notch being present in the tym- 

 panic frame of such forms (the Chelydridio, PlatystcrnidcC, and land 

 Tortoises) as ha\ e the outer ear-chanibcr closed behind ; in other 

 forms, especially the marine Turtles, the stapes are completely, or 

 nearly comj)li'tely, exposed behind. In the Chclonidtc, Chelydridae, 

 Dcrmatcmydidie, Cinosternida% and Platysternida; the tympanic 

 frame is not formed by the quadrate alone, as in the Testudinidae, 

 but also by the squamosal and the quadrato-jugal. 



The palatal surface is characterized by the comparative narrow- 

 ness of the pterygoids, the outer borders of which are concavely 

 arched or convergent behind a more or less distinct process which 

 apparently answers to the ectopterygoid process of the lihyncho- 

 cephalia and Lacertilia ; this character is most marked in Macro- 

 clemmijs, least in the land Tortoises. In the latter the palate is 

 deeply concave, and the vomer develops a more or less strong 

 median keel or ridge which, in Testwlo poh/i)hcmus and calrarata, 

 is produced far back between the pterygoids. A foramen, analogous 

 to the palato-pterygoid, is present in all forms save the Chelonidae. 

 The basisphenoid is separated from the (luadrate by the pterj-goids, 

 which form a suture with the basioccipital, or very nearly reach the 

 latter bone, and are in contact with the maxillaries (except in the 

 Chelonidae); the vomer, which is single, forms a septum between 

 the choanoj, and joins the praemaxillaries on the palate, separating 

 the maxillaries (except in some specimens of Tludassocheli/s). 



The mandible contains a single dentary and five paired elements, 

 viz. the coronoid, the opercular, and the angular on the inner side, 

 the supraangular on the outer side, and, between the opercular and 

 the supraangular, the articular, which is small and with a concave 

 surface. 



Htoid Arch. — The body is short, ossified in one, three, or 

 four parts in the freshwater forms ; entirely cartilaginous or with 

 two or four small ossifications in the terrestrial and marine. The 

 entoglossal, which is situated below the anterior extremity of the 

 body of the hyoid, is small and cartilaginous. In most freshwater 

 Cryptodira three pairs of cornua are present — the anterior very 

 small and cartilaginous, the median largest, bony, and with a carti- 

 laginous epiphysis, the posterior similar to the median but shorter ; 

 in the Chelouid;e there are likewise three pairs of cornua, but the 

 posterior are cartilaginous ; in the land Tortoises and in jYicoria 

 there are no distinct anterior cornua, and the posterior are but 

 cartilaginous. 



Pectoral Arch and Fore Limb. — Of the three branches which 

 constitute the pectoral arch, viz. the scapula, the prtecoracoid, and 

 the coracoid, the latter is the longest in the Chelonidae, the former 

 in all other Cryptodira. In the land Tortoises the coracoid is much 

 expanded, subtriangular, its width nearly equalling its length. The 



c 



